1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method of transmitting feedback information from an user equipment that performs CoMP operation.
2. Discussion of the Related Art
Recently, a multiple input multiple output (MIMO) system has received much attention as a broadband wireless communication technology. The MIMO system means a system that enhances communication efficiency of data by using a plurality of antennas. The MIMO system can be divided into a spatial multiplexing scheme and a spatial diversity scheme depending on transmission of same data.
The spatial multiplexing scheme means a scheme that can transmit data at high rate without even increasing a system bandwidth by simultaneously transmitting different data through a plurality of transmitting antennas. The spatial multiplexing scheme means a scheme that can obtain transmission diversity by transmitting same data from a plurality of transmitting antennas. An example of the spatial diversity scheme includes a space time channel coding scheme.
Also, the MIMO system can be divided into an open loop system and a closed loop system depending on feedback of channel information from a receiving side to a transmitting side. An example of the open loop system includes a space-time trellis code (STTC) system in which a transmitting side transmits information in parallel while a receiving side detects a signal by repeatedly using a zero forcing (ZF) scheme and a minimum mean square error (MMSE) scheme and obtains transmission diversity and encoding gain by using a blast and spatial area that can increase information as much as the number of transmitting antennas. An example of the closed loop system includes a transmit antenna array (TxAA) system.
A coordinated multi-point system (hereinafter, referred to as ‘CoMP system’) is the system for improving throughput of a user located at the cell edge by applying improved MIMO transmission under the multi-cell environment. If the CoMP system is applied, inter-cell interference can be reduced under the multi-cell environment. In case of the CoMP system, a user equipment can commonly be supported with data from multi-cell base stations.
Also, each base station can improve system throughput by supporting same radio frequency resource to one or more user equipments (UE1, UE2, . . . , UEK) at the same time. Also, the base station can perform a space division multiple access (SDMA) method based on channel status information (CSI) between the base station and the user equipment.
The CoMP system can be divided into a coordinated MIMO (Co-MIMO) type joint processing (JP) scheme through data sharing and a coordinated scheduling scheme/beamforming (CS/CB) scheme.
FIG. 1 is a conceptional diagram illustrating a CoMP system of intra eNBs and an inter eNB according to the related art.
Referring to FIG. 1, intra base stations 110, 120 and inter base station 130 exist under a multi cell environment. According to a long term evolution (LTE), the intra base stations include several cells (or sectors). Cells that belong to a base station to which a specific user equipment belongs are the relation of intra base stations 110, 120 with a specific user equipment. Namely, cells that share a base station to which a user equipment belongs are cells corresponding to the intra base stations 110 and 120 while cells that belong to other base stations are cells corresponding to the inter base station 130. In this way, although cells based on the same base station as that of a specific user equipment transmit and receive information (for example, data, channel status information (CSI)) through 2 interface, cells based on a base station different from that of a specific user equipment can transmit and receive information through a backhaul 140.
As illustrated in FIG. 1, a single cell MIMO user 150 located within a single cell performs communication with a single serving base station in one cell (sector), and a multi-cell MIMO user 160 located at the cell edge performs communication with a plurality of serving base stations in multiple cells (sectors).
In order to preferably perform the aforementioned CoMP operation, the user equipment should transmit much feedback information to the base station. Such a method for transmitting feedback information according to the related art has a problem in that overhead of uplink resources increases due to feedback transmission. Also, overhead of backhaul and delay due to excessive exchange of feedback information may cause a significant problem for the system.